Spinal implant and method for restricting spinal flexion

ABSTRACT

A spinal implant system for restricting flexion of a spine includes an elongate band proportioned to engage at least two spinous processes. During use, the band is positioned engaging the spinous processes at a spinal segment of interest, where it restricts flexion at the segment. The length and tension of the band may be adjustable following to implantation using percutaneous or transcutaneous means.

PRIORITY

This application claims the benefit of U.S. Provisional Application No.60/551,235, filed Mar. 9, 2004, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to the field of spinal implantsand more particularly to the field of implants for restricting spinalflexion.

BACKGROUND OF THE INVENTION

A major source of chronic low back pain is discogenic pain, which isalso known as internal disc disruption. Patients suffering fromdiscogenic pain tend to be young (30-50 years of age), otherwise healthyindividuals who present with pain localized to the back. Usuallydiscogenic pain occurs at the discs located at the L4-L5 or L5-S1junctions. Pain tends to be exacerbated when patients put their lumbarspines into flexion (i.e. by sitting or bending forward) and relievedwhen they put their lumbar spines into extension (i.e. archingbackwards). Discogenic pain can be quite disabling, and for somepatients it dramatically affects their ability to work and otherwiseenjoy their lives.

Current treatment alternatives for patients diagnosed with chronicdiscogenic pain are quite limited. Many patients continue withconservative treatment (examples include physical therapy, massage,anti-inflammatory and analgesic medications, muscle relaxants, andepidural steroid injections) and live with a significant degree of pain.Others elect to undergo spinal fusion surgery, which typically involvesdiscectomy (removal of the disk) together with fusion of adjacentvertebrae. Fusion is not typically recommended for discogenic painbecause it is irreversible, costly, associated with high morbidity, andof questionable effectiveness. Despite its drawbacks, however, spinalfusion for discogenic pain remains common due to the lack of viablealternatives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a portion of a human lumbar spineillustrating the spine under normal loading.

FIG. 1B is a schematic view similar to FIG. 1A illustrating the spine inflexion.

FIG. 2 is a schematic view similar to FIG. 1A illustrating a firstembodiment of a spinal implant positioned around adjacent spinousprocesses.

FIG. 3A is a perspective view of the spinal implant of FIG. 2 in anopened position.

FIG. 3B is a perspective view of the spinal implant of FIG. 2 in aclosed position.

FIG. 3C is a side elevation view of the spinal implant of FIG. 2 in anopened position, and slightly modified to include motorizedtension/length adjustment and an extracorporeal transmitter forcontrolling tension/length adjustments.

FIG. 3D is a perspective view of an implant similar to the implant ofFIG. 2, but using a turnbuckle style connector.

FIG. 3E is an exploded perspective view of another implant similar tothe implant of FIG. 2, but using yet another alternative connectorarrangement.

FIGS. 4A through 4D are a sequence of views of a human lumbar spineillustrating a sequence of steps for implanting the spinal implant ofFIG. 2.

FIG. 5A is a perspective view of the FIG. 2 embodiment illustrating anaccess window for a worm screw adjustment mechanism.

FIG. 5B is a perspective view of an adjustment tool engageable withimplant of FIG. 2 for using in adjusting the effective length of theband.

FIG. 6 is a schematic illustration of a human lumbar spine illustratinguse of the adjustment tool of FIG. 5B on the spinal implant of FIG. 5A.

FIG. 7 is a perspective view of components of a third embodiment of aspinal implant.

FIGS. 8A through 8D illustrate a sequence of steps for implanting theembodiment of FIG. 7.

FIG. 9 is an exploded view of a housing assembly for a compressionmember of the embodiment of FIG. 7.

DETAILED DESCRIPTION

FIG. 1A illustrates a portion of the lumbar region of a human spine. Asshown, the spine includes five lumbar vertebrae 2. Each of the vertebrae2 has a vertebral body 4, lateral projections known as transverseprocesses 8, and a posterior projection called the spinous process 10.The vertebral bodies 4 are separated from one another by intervertebraldiscs 12. Each disc includes a nucleus pulposus, a semi-gelatinoustissue in the center of the disc surrounded and contained by the highlyinnervated annulus fibrosus, a circumferential band of tissue thatprevents this material from protruding outside the disc space.

Under normal loading the lumbar spine curves forward as shown in FIG.1A. In this condition, the discs 12 are subjected to axial forces by thevertebral bodies. Accordingly, the nucleus pulposus is undercompression, while the annulus fibrosis experiences circumferential hoopstresses. However, when the lumbar spine is placed in flexion as shownin FIG. 1B (such as when the patient bends forward or sits), the discexperiences axial forces as well as forward bending forces. The bendingforces displace nucleus polposus material dorsally, causing the anteriorportion of the disc to be slightly compressed while the posteriorportion of the disc is slightly expanded as shown. As a result, thedorsal region of the annulus is placed in tension, causing the patientto experience pain.

As can be seen by comparing FIGS. 1A and 1B, the spinous processes 10spread apart from one another when the lumbar spine is under flexion.Several of the embodiments described herein restrict this spreading as ameans for restricting flexion. In particular, FIGS. 2-8A illustrateembodiments of spinal implants for coupling two or more spinousprocesses of the lumbar vertebra as a means for restricting flexion ofthe lumbar spine so as to minimize lumber pain. It should be noted thatalthough these embodiments are described in the context of the lumbarspine, they may be equally useful in the cervical spine. Moreover, thedevices and methods described herein may be adapted for use in otherareas of the spine and elsewhere in the body. For example, the devicesmay be implanted at alternative locations in the body where they mayfunction as artificial ligaments, and the adjustment methods describedherein may be used for adjusting artificial ligaments.

Referring to FIG. 2, a first embodiment of a spinal implant comprises anelastic or semi-elastic band 20 sized to be positionable around adjacentspinous processes 10. Once implanted, the band 20 limits the amount ofspreading between spinous processes 10 upon spinal flexion. Thus, whenthe patient bends the band 20 is placed in tension, thereby restrictingposterior tension that would otherwise be experienced by the dorsalannulus and limiting the amount of flexion that can be achieved. Thus,the intervertebral disc is subjected to limited bending forces and painis therefore reduced or eliminated. In this capacity, the band functionsas an artificial ligament, supplementing the supraspinous andinterspinous ligaments, which are normally loaded during flexion.

As best shown in FIG. 3A, band 20 includes an elongate portion 22 havingends 24 a, 24 b and a connector 26 for coupling the ends 24 a, 24 b toform the band into a loop. The connector 26 includes a housing 27 havinga keyway opening 28 at end 24 a, and a threaded rod or screw 30extending from end 24 b. It should be noted that in FIGS. 3A and 3B theband is partially twisted to cause keyway opening 28 to face outwardlyso that it may be more easily seen and described. However, when the bandis implanted the connector preferably faces inwardly as shown in FIGS.3C, 5A and 6.

Threaded rod 30 includes a broadened head 32 that is receivable in thekeyway opening 28. To engage the ends 24 a, 24 b, broadened head 32 isinserted into the keyway opening as indicated by arrow X, and then drawnupwardly as indicated by arrow Y to seat broadened head 32 within thehousing 27 as shown in FIG. 3B. In an alternative embodiment, the rod 30may be replaced by an alternative rod (not shown) that extendslongitudinally from end 24 b for streamlined implantation but thatincludes an end portion pivotable laterally to form a “T” shaped member.To close the loop, the cross member of the “T” is moveable intoengagement with a keyway or other receiver on end 24 a.

The connecting mechanism 26 illustrated in FIGS. 3A and 3B isparticularly useful in that it allows the effective length of the bandto be adjusted as needed by the surgeon. Referring again to FIG. 3A, thethreads of rod 30 are engaged within corresponding threads of a captivenut 34 retained within a bore 36 in a housing 29 at connector end 24 b.The rod 30 may be longitudinally advanced or retracted within the bore36 by axially rotating the rod in a clockwise or counter-clockwisedirection, such as by manually turning the head 32 using the thumb andforefinger. This allows the physician to adjust the effective length anddegree of tension of the implant 20 prior to and/or after implantation.In an alternative embodiment, the device may include multiple adjustmentassemblies to allow for gross adjustment as well as fine tuning of thedevice length and tension. Moreover, the implant may be formed ofmultiple elements (e.g. having multiple straps, connectors, and/oradjustment mechanisms) that come together to form the band.

Various other types of connecting mechanisms may be used to connect theends of the band to form a loop. In an alternative embodiment shown inFIG. 3D, a turnbuckle nut 21 has internal threads engageable withthreaded members located at the free ends of band 20 a. The free ends ofthe band 20 a include opposite thread patterns so that rotation of theturnbuckle in one direction draws the ends towards one another toshorten the loop, whereas rotation in the opposite direction spreads theends away from one another to lengthen the loop. Other types ofconnecting mechanisms that may be used to form the band into a loopinclude hooks, crimp, zip tie type configurations, housing and set screwassemblies, hand-tied or instrument-tied knots or any other form ofsuitable connector.

Another example, illustrated in exploded perspective view in FIG. 3E,includes a connector 26 b having two threaded worm gears 27 a, 27 b anda worm screw 31. The threaded worm gears 27 a, 27 b receivecorresponding threaded rods 29 a, 29 b, each of which is coupled to anend of the band 20 b as shown. When implanted, the band 20 b ispreferably oriented such that connector 26 b is positioned above thesuperior spinous process (as shown) or below the inferior spinousprocess and such that the rods 29 a, 29 b extend in a generally axialdirection next to and parallel to the sides of the spinous process. Theworm screw and the threaded worm gears are arranged such that rotationof the worm screw in one direction effectively lengthens the band 20 bby causing the worm gears to move along the rods in a superior direction(see arrow S), and such that rotation of the worm screw in the oppositedirection effectively shortens the band by causing the worm gears tomove along the rods in an inferior direction (see arrow I). Thus, theconnector 26 b allows a single worm gear within the connector assemblyto simultaneously adjust both ends of the band to tighten or loosen thefull band. The connector is preferably partially enclosed within ahousing (not shown), with worm screw 31 exposed for access by a user.

Although in the FIG. 3A through FIG. 3E embodiments the ends of the bandinclude components that engage with one another to form the loopconnection, other configurations are possible. For example, the ends ofthe band may be drawn together and anchored together using a separatecoupling device. A spring-loaded cord lock having a hole which receivesthe ends of band and which allows the band to be “cinched” around thespinous processes is one example. As another example, a single end ofthe band may include a coupling device which can receive and engage theopposite free end of the band. Naturally, many alternativeconfigurations are conceivable.

Referring again to the preferred embodiment of FIG. 3A, the elongateportion 22 of the band 20 is preferably made of a biocompatible elasticor semi-elastic material selected to restrict flexion of the lumbarspine by a desired degree (e.g. an amount sufficient to relieve pain) byincreasing the stiffness of the effected joint segment (e.g. L4/L5) bysome amount., which may vary for different patients. In one embodiment,the spring constant of the device would be 25-75N/mm. In a preferredembodiment, flexion is not completely inhibited, but is instead limitedonly within the range in which flexion causes pain or significant pain,while rotation and extension are incidentally and only minimallyrestricted by placement of the band. Suitable examples of materialsinclude (but are not limited to) single monofilament elastic materialsin tension (e.g. elastomers or suitable biocompatible materials such asthose manufactured by Polymer Technology Group of Berkeley, Calif.),polyurethanes, polycarbone/polyether urethanes, thermoplastic siliconeurethane copolymers, metal springs (e.g. stainless steel or NiTiNOL,coiled or otherwise, in tension or compression), knit or wovenmaterials. If desired, the interior surface of the band may haveproperties for preventing slipping between the band and adjacent bone,and/or for promoting ingrowth of bone material or other tissue ingrowththat facilitates retention of the band. For example, the interiorsurface of the band may include gripping elements or surface roughness,or a porous in-growth-promoting surface. Alternatively, supplementalelements may be combined with the band 20 to prevent slipping, such asthe elements 23 shown in FIG. 3D. Such elements may be pre-attached tothe band or separately placed in contact with the spinous process duringimplantation to prevent slipping.

In an alternative embodiment, suitable bioresorbable materials mayalternatively be used for some or all of the components.

One method for implanting the band 20 of FIG. 2 will next be described.Implantation may be carried out using surgical techniques via a midlineincision, or minimally invasively through a keyhole incision. Afterpenetration through the fascia, the muscle next to the spinous processesis distracted to give the surgeon access to the spinous processes andinterspinous ligaments. Two holes may be pierced in the interspinousligaments to facilitate implantation of the device: one in theinterspinous ligament above the superior spinous process and one in theinterspinous ligament below the inferior spinous process. Prior topositioning of the band 20, the effective length of band may optionallybe adjusted by rotating threaded rod 30 to increase or decrease itseffective length as described above. FIGS. 4A through 4C illustrate oneexample of a method for implanting the band 20. According to thisexemplary method, one end 24 a of the band 20 is passed through theincision and advanced superiorly until it is positioned adjacent to thedesired pair of spinous processes 10 a, 10 b. FIGS. 4A and 4B. One orboth ends of the band 20 are wrapped around the spinous processes 10 a,10 b and brought towards one another as shown in FIG. 4C. If the spinalsegment of interest is one at which there is a spinous process that isinadequate or otherwise shaped in a manner that will prevent it fromretaining the band, the spinous process may be engaged by the band in analternative way. For example, a small hole may be formed in that spinousprocess and the band may be passed through the small hole. This approachmay be particularly desirable where a band is to be placed at the L5-S1segment.

The head 32 is inserted into the keyway opening to engage the ends 24 a,24 b. If the surgeon determines that the band 20 is too loose toadequately restrict flexion or too tight to permit an appropriate amountof flexion, adjustments to the effective length of rod 30 may be made atthis time. As is clear from the description, the band is preferably heldin place without the use of screws, nails, or other attachment devicespenetrating the bone or cemented to the bone—although in somecircumstances it may be desirable to utilize such means of attachment tofacilitate retention.

As discussed above, the implanted band 20 limits the amount of spinalflexion without significantly restricting rotation or extension of thespine. If necessary for the patient's condition, multiple bands may beused at multiple segments within the spine.

As discussed previously, band 20 preferably includes features foradjusting the effective length of the band, such as by rotating thethreaded rod 30 to cause it to screw further into or out of the captivenut 34. It is further preferable to provide features that permit thistype of adjustment both immediately following implantation and in alater procedure if desired.

Referring to FIG. 5A, the broadened head 32 of the threaded rod 30 maytake the form of a worm gear having teeth along its exterior edges. Whenthe band is assembled, a portion of the worm gear 32 is exposed througha window 38 in housing 27. Referring to FIG. 5B, a suitable adjustmenttool 50 includes an elongate rod 52 and a threaded member 54 that isengageable with the exposed threads on the worm screw 32. An L-shapedspring clip 56 extends laterally from the rod 52.

Referring to FIG. 6, to adjust the length of the band 20, the tool'sthreaded member 54 is engaged against the corresponding threads of theworm gear 32, while the spring plate 56 is positioned in contact with anopposite side of the housing 27 to steady the tool 50 against theimplant. When the threaded member of the adjustment tool is turned, itrotates the worm gear 32, causing the threaded rod 30 to move up or downthrough the captive nut, thereby lengthening or shortening the effectivelength of the band as shown in FIG. 6. As discussed, this adjustment maybe carried out immediately following positioning of the band 20, orduring a separate procedure at a later date if it is determined thatincreased or decreased restriction is desired. The tool 50 could bethreaded through a small incision to gain access to the implanteddevice.

Naturally, many other mechanisms for adjusting the effective length ofthe band before and/or after implantation may be provided. Thesemechanisms may be operatively associated with the band's connectingmechanism, or they may be entirely separate from the band's connectingmechanism. One example includes the turnbuckle arrangement shown in FIG.3B, in which the turnbuckle nut 21 may be rotated in one direction totighten the loop or rotated in the other direction to loosen the loop.Other examples include hose-clamp type mechanisms, ratchetconfigurations or screw mechanisms that may be adjusted using adjustmenttools (e.g. screwdrivers, ratcheting devices etc.), piston elements thatexpand or contract upon addition or removal of a fluid (e.g. air orliquid) into or from a reservoir using a syringe, shape memory elementsor other elements having material properties that will allow for aneffective change in response to changed environmental conditions (e.g.electrically or thermally activated length changes initiated byapplication of heat or electricity using a conductor positioned adjacentto or into contact with the band, or chemically activated length changesusing a chemical agent passed into contact with the band using a syringeor catheter). Links or other elements that may be added/removed toincrease/decrease the band's length, or lengths of elastic materialforming the band may be wrapped on a spool and payed out or coiled up toeffect length changes in the band. As another example, sections of theband material may be displaced transversely to shorten the effectivelength of the band.

Referring to FIG. 3C, the band 20 may be equipped with a motor 33 andbattery 35 for effecting band length changes by driving the rod 30,together with electronic components 37 that control the motor. Althoughone arrangement of components is illustrated in FIG. 3C, many others arepossible, including one in which the motor drives the head 32 of thethreaded rod 30.

Additional electronics 39 may allow length changes to be accomplishednon-invasively. For example, the electronic components 39 may includereceivers for radio, optical, or magnetic signals transmitted throughthe skin. These components may be housed within housing 29 a with themotor coupled to threaded rod 30. The device would thus be programmed toadjust the band length in response to transcutaneous signals receivedfrom an external device 41. The band's electronics 39 may furtherinclude transmitters for transmitting data indicating the current lengthand/or tension setting for the implant to the external device 41. In asimilar embodiment, the band may be coupled to one or more buttonspalpable through the skin that can activate the motor for lengthadjustments. The buttons may be positioned directly on the band or in asubcutaneous pocket. In other embodiments, the band may be manuallyadjusted by manipulating an actuator (e.g. a rotatable knob) palpablethrough the skin. In other embodiments, the band may be adjusted bypassing a magnet over the skin in order to mechanically manipulate anelement under the skin via the magnetic field. In other embodiments, amotorized pump may pump fluid to or from a piston chamber to affect achange in loop length. Transcutaneous length adjustments are alsopossible using embodiments of the type above for which a syringe may beused as an adjustment tool (e.g. for injecting fluid into a pistonchamber or for injecting a chemical agent into contact with a chemicallymaterial that changes shape in response to chemical exposure), bypassing the syringe through the skin to the target location. A tatoo orother visual marker may be positioned on the skin to allow the physicianto easily identify the target injection site.

Components of a third embodiment of a spinal implant 70 are illustratedin FIG. 7. Implant 70 is similar to the prior embodiments in that, whenassembled, it takes the form of a band positionable around the spinousprocesses. However, it differs from prior embodiments in that itrestricts spinal flexion using compression elements that are placed incompression upon expansion of the band during spinal flexion.

Referring to FIG. 7, implant 70 includes a pair of elastomeric members72 a, 72 b, each of which includes a throughbore 74 a, 74 b and a cable76 a, 76 b extending through the throughbore. Cables 76 a, 76 b arepreferably inelastic, although in an alternative embodiment elasticcables may also be used. The elastomeric members may be housed in anenclosure or casing 86 (FIG. 9) formed of a biocompatible metal (e.g.titanium or stainless steel) or polymer, if desired. Casing 86 includesa removable cap 88 which mates with the casing using threadedconnections. Openings 89 a, 89 b are provided in the casing 86 and cap88 for passage of the cables 76 a, 76 b.

Each cable preferably includes a first stopper element 78 a, 78 b fixedto one of its ends, and a second stopper element 80 a, 80 b attachableto its free end. If the casing 86 of FIG. 9 is used, a washer 90 may bepositioned between each stopper element 78 a and its correspondingelastomeric member 72 a.

The stopper elements 78 a, 78 b, 80 a, 80 b are proportioned such thatthey cannot pass through the throughbores 74 a, 74 b. The implant 70further includes a pair of arcuate elements 82 a, 82 b, each of whichincludes receiving elements such as loops 84 a, 84 b on its convex sideas shown. Each of the cables 76 a, 76 b extends through the loops of oneof the rigid arcuate elements 82 a, 82 b. During use, the arcuateelements 82 a, 82 b are positioned in contact with the spinous processeswhere they may prevent slipping between the band and adjacent bone. Theelements 82 a, 82 b may also utilize materials or coatings that willpromote tissue ingrowth that facilitates retention of the band. Theelements 82 a, 82 b may also provide a track through which the bandpasses. Although the elements 82 a, 82 b are shown as being arcuate,they may have other shapes without departing from the scope of theinvention.

A method for positioning the implant 70 will next be described withreference to FIGS. 8A through 8D. For simplicity, only the posteriorsides of the spinous processes 10 involved in the procedure are shown.Prior to implantation, each cable is preferably preassembled with itscorresponding arcuate element and elastomeric member. Referring to FIG.8A, a first one of the cables 76 a is positioned with its arcuateelement 82 a positioned with its concave side facing a spinous process10 a. The second cable 76 a is similarly introduced at the other spinousprocess 10 b, and the free end of each cable 76 a, 76 b is threadedthrough the elastomeric member 72 a, 72 b of the opposite cable as shownin FIG. 8B. Finally, the remaining stoppers 80 a, 80 b are mounted tothe free ends of the cables as shown in FIG. 8C, causing each of theelastomeric members to be positioned between and in contact with a pairof the stoppers. The cables are tightened around the spinous process asshown in FIG. 8D, and the stoppers are locked in place (such as bycrimping or other action) to retain the cables in the tightenedconfiguration.

As discussed previously, flexion of the spinal segment will cause thespinous processes 10 a, 10 b to spread away from one another. When theimplant 70 is positioned as shown in FIG. 8D, spreading of the spinalprocesses will impart expansive forces to the implant 70. Theseexpansive forces cause each of the elastomeric members 72 a, 72 b to becompressed between a pair of the stoppers 80 a, 80 b, 78 a, 78 b,thereby restricting expansion of the implant 70 and thus limiting spinalflexion at the segment in which the implant is positioned. Variousembodiments of systems, devices and methods have been described herein.These embodiments are given only by way of example and are not intendedto limit the scope of the present invention. It should be appreciated,moreover, that the various features of the embodiments that have beendescribed may be combined in various ways to produce numerous additionalembodiments. Moreover, while various materials, dimensions, shapes,implantation locations, etc. have been described for use with disclosedembodiments, others besides those disclosed may be utilized withoutexceeding the scope of the invention.

1. A method of restricting flexion of a spine, comprising the steps of:providing an elongate band having first and second ends; positioning thestrap to engage a pair of spinous processes; and coupling the first andsecond ends to form the band into a loop engaging the pair of spinousprocesses.
 2. The method according to claim 1, wherein the methodincludes the step of adjusting the effective length of the band.
 3. Themethod according to claim 2, wherein the adjusting step is performedprior to the coupling step.
 4. The method according to claim 2, whereinthe adjusting step is performed following the coupling step.
 5. Themethod according to claim 4, wherein the positioning and coupling stepsare performed in a first procedure, and the adjusting step is performedin a second, separate, procedure.
 6. The method according to claim 4,wherein the adjusting step includes the step of introducing anadjustment tool percutaneously into contact with the band and using theadjustment tool to adjust the effective length.
 7. The method accordingto claim 4, wherein the adjusting step includes the step of positioninga transcutaneous adjustment device outside the patient in proximity tothe band, and using the adjustment device to control adjustment of theeffective length.
 8. The method according to claim 7, wherein thetranscutaneous adjustment device is selected from the group consistingof telemetric, magnetic, optical, and radiofrequency control devices. 9.The method according to claim 1, wherein the coupling step causes theband to restrict flexion of the spine in the region of the band.
 10. Themethod according to claim 1, wherein the band allows substantiallyunrestricted rotation and extension of the spine.
 11. The methodaccording to claim 1, wherein at least a portion of the band is intension during flexion of the spine.
 12. The method according to claim1, wherein at least a portion of the band is under compression duringflexion of the spine.
 13. The method according to claim 12, wherein: theproviding step provides the elongate band to include a first bandelement having the first end thereon and a first compression member onthe first band element, and a second band element having the second endthereon, and a second compression member on the second band element; andthe coupling step includes coupling the first end to the secondcompression member and coupling the second end to the first compressionmember.
 14. The method according to claim 13, wherein the coupling stepincludes threading the first end through an opening in the secondcompression member and threading the second end through an opening inthe first compression member.
 15. A spinal implant system forrestricting flexion of a spine, the implant comprising: an elongate bandproportioned to encircle at least two spinous processes, and at leastone connector engageable with a portion of the band to retain the bandin a loop engaging the spinous processes.
 16. The implant systemaccording to claim 15, wherein the band is at least partially elastic.17. The implant system according to claim 15, wherein the system furtherincludes implantation instructions describing a method for positioningthe band around at least two spinous processes and engaging theconnector with a portion of the band to form the band into a loop. 18.The implant system according to claim 15, wherein the band includes anadjustment member extendable and retractable to adjust the effectivelength of the band.
 19. The implant system according to claim 16,wherein the adjustment member includes a threaded rod rotatable in afirst direction to lengthen the band and rotatable in a second directionto shorten the band.
 20. The implant system according to claim 19,wherein the connector includes the threaded rod, wherein the threadedrod includes a head, and wherein the band includes a keyholeproportioned to receive the head.
 21. The implant system according toclaim 20, wherein the head includes external teeth, and wherein thesystem includes an adjustment tool engageable with the external teeth torotate the threaded rod.
 22. The implant system according to claim 15,wherein the system includes an adjustment tool engageable with the bandto selectively modify the length of the band.
 23. The implant systemaccording to claim 15, wherein the elongate band is expandable uponflexion of the spine, and wherein expansion of the elongate band placesat least a portion of the elongate band under tension.
 24. The implantsystem according to claim 15, wherein expansion of the elongate bandplaces at least a portion of the elongate band under compression. 25.The implant system according to claim 24, wherein; the connectorincludes a first connector portion and a second connector portion; theelongate band includes a first band element having the first connectorportion thereon and a first compression member on the first bandelement, and a second band element having the second connector portionthereon, and a second compression member on the second band element; andthe first connector portion is coupled to the second compression memberand the second connector is coupled to the first compression member toform the loop.
 26. The implant system according to claim 15, wherein thesystem further includes a compression member positioned such thatexpansion of the elongate band places the compression member undercompression.
 27. The implant system according to claim 18, wherein thesystem further includes a motor coupled to the adjustment mechanism, acontroller electronically coupled to the motor, and an extracorporealtransmitting device, the controller responsive to transcutaneous signalsfrom the transmitting device to activate the motor.
 28. The implantsystem according to claim 27, wherein the extracorporeal transmittingdevice is selected from the group consisting of telemetric, magnetic,optical, and radiofrequency control devices.
 29. A method for adjustingan artificial ligament, comprising the steps of: implanting anartificial ligament within a living body, the artificial ligamentincluding an elongate band having an extendable and retractable member;and adjusting the length of the band by positioning a transcutaneousadjustment device outside the patient, and using the adjustment deviceto direct extension or retraction of the extendable and retractablemember.